For the control of concentrations, which cannot be measured directly on-line in thermal separating processes, in many cases temperatures are used as substitute controlled variable. To obtain a sufficiently good correlation with the controlled variable "concentration" in the past the concept of pressure-compensated temperature has frequently been employed, where the temperature changes caused by pressure fluctuations are taken into consideration and the remaining temperature changes represent a better gauge of the concentration to be controlled (see, for example F. F. Rhiel, R. Perne, S. Pauen and F. Staffaris, "Model-Based Control Systems for Distillation Columns", Maastrich, DYCORD+ '89, pages 31 to 35).
This concept has found useful applications in particular in the field of controlling sump concentrations in distillation columns. However, due to a limited operating range and restriction to pressures above the normal pressure, this control method can unfortunately be used only to a limited extent.
In general the problem of achieving a constant, controlled reaction in the production of polymers applies to all polymerization-, polycondensation- and polyaddition processes in which process-related fluctuations in pressure and temperature and disturbances in pressure and temperature influence the constancy of the quality of the polymer. Conventional control concepts generally possess the disadvantage that they recognise the process trend too late and thus do not permit prompt intervention into the production process by means of correcting variables.
An attempt has been made to overcome some of these disadvantages by the method which has been described in DE 36 27 287 and in which, in particular for the continuous production of polyvinyl chloride, the internal temperature of the reactor is controlled by the quantity of cooling water, and the inflowing quantity of activator solution is controlled by the cooling water reflux temperature, whereby the polymerization temperature is maintained constant at a fixed value and the polymerization pressure always remains below the saturation pressure of the vinyl chloride and is varied by fine adjustment of the inflowing quantity of liquid vinyl chloride.